In systems for optical information transmission, there is the need of coupling transmission and reception means to the system transmission line when the line is in operation. Connection devices, such as described in U.S. Pat. No. 4,252,402, can be coupled in at desired points along the line in such a case. During the time coupling is in progress, the traffic on the line must be interrupted, and this can result in large costs. To avoid these costs, a large number of connection devices are already coupled in beforehand when the transmission line is constructed, and are often considerably more than the line can serve at the same time. This is also expensive, and it is difficult to anticipate suitable connection points along the transmission line. The known connection devices further have the disadvantage that possible crosstalk in the optoelectric directional switches included in the connection devices allow the entire crosstalk signal to pass out from the device.